1. Field of the Invention
The present invention relates to an acidic aqueous phosphate solution and a process for phosphating a metal surface with said solution. More particularly, it relates to a solution and a process for forming a phosphate film especially suitable for cationic electrocoating, and is particularly applicable to metal surfaces which include an iron-based surface and a zinc-based surface such as an automobile body.
2. Description of Related Art
Japanese Patent Publication (unexamined) No. 107784/1980 (laid-open to the public on Aug. 19, 1980) discloses a process for treating a metal surface by dip treatment, followed by spray treatment, with an acidic aqueous phosphate solution containing from 0.5 to 1.5 g/l of zinc ion, from 5 to 30 g/l of phosphate ion, and from 0.01 to 0.2 g/l of nitrite ion and/or from 0.05 to 2 g/l of m-nitrobenzenesulfonate ion. Said process is reported to be capable of providing a phosphate film which is effective for forming a coating by cationic electrocoating having excellent adhesion and corrosion-resistance on complicated articles having many pocket portions like car bodies.
Japanese Patent Publication (unexamined) No. 145180/1980 (laid-open to the public on Nov. 12, 1980) discloses a process for treating a metal surface by spray treatment with an acidic aqueous phosphate solution containing from 0.4 to 1.0 g/l of zinc ion, from 5 to 40 g/l of phosphate ion, from 2.0 to 5.0 g/l of chlorate ion, and from 0.01 to 0.2 g/l of nitrite ion. Further, Japanese Patent Publication (unexamined) No. 152183/1980 (laid-open to the public on November 27, 1980) discloses an acidic aqueous phosphate solution containing from 0.08 to 0.20 wt. % of zinc ion, from 0.8 to 3.0 wt. % of phosphate ion, from 0.05 to 0.35 wt. % of chlorate ion, from 0.001 to 0.10 wt. % of nitrite ion, and complex fluoride ion in an amount calculated by the formula: 0.4.gtoreq.y.gtoreq.0.63x-0.042, wherein x is the concentration in wt. % of zinc ion and y is the concentration in wt. % of the complex fluoride ion. These prior art processes are reported to be capable of providing excellent adhesion and corrosionresistance to the coating by cationic electrocoating.
However, in a recent development in the automobile industry there has come to be used for car bodies steel components plated on one surface only with zinc or alloyed zinc, with the object of further improving corrosion-resistance after the application of the siccative coating. It has however come to be recognized that, when the above prior art compositions and processes are applied to such materials (i.e. to metal surfaces which include both an iron-based surface and a zinc-based surface), on the iron-based surface a phosphate film suitable as a substrate to be coated by cationic electrocoating can be formed as desired, but a phosphate film formed on a zinc-based surface is significantly inferior to that formed on the iron-based surface.
There has been developed a composition and process to solve the above-mentioned problems which occur on zinc-based surfaces in components which include both an iron-based surface and a zinc-based surface. This is the invention disclosed in Japanese Patent Publication (unexamined) No. 152472/1982 (laid-open to the public on Sep. 20, 1982). Said invention is characterized by subjecting the metal surface to treatment with an acidic aqueous phosphate solution comprising from 0.5 to 1.5 g/l of zinc ion, from 5 to 30 g/l of phosphate ion, from 0.6 to 3 g/l of manganese ion, and a phosphating accelerator.
However, in the zinc phosphate treatment of galvanized steel, there often results an abnormal coating with white spots, each spot having a diameter of from 1 mm to 2 mm. This is particularly true with an electroplated galvanized steel and especially in the dip treatment thereof. These white spots will cause craters in subsequent treatment, thereby resulting in inferior coatings. The mechanism by which white spots are formed is believed to be as follows: In a first stage, there appear many pits, at the edge portions of which the galvanized layer is gradually dissolved in the form of concentric circles through an excessive etching reaction. As the growth of each pit continues, zinc phospate is precipitated in the center portion thereof, However, at the peripheral portions, the iron surface is exposed, which forms a galvanic cell with the zinc metal, thereby continuing the dissolution of the zinc.
As a consequence, an excess quantity of zinc phosphate crystals are precipitated and accumulate as "snow" at the peripheral portions of the spots, which can readily be observed by the naked eye.
Unfortunately, no solution to this problem has been found heretofor to consistently avoid the formation of such white spots.